Wet mix molding processes are processes which utilize an unthickened, i.e., liquid molding composition. The molding composition is typically introduced into a mold which contains a substrate such as, for example, a fiberglass mat, and is molded, e.g., by compression molding, to produce a desired article.
Typical wet mix molding processes include, for example, pultrusion molding, preform molding and wet mat molding. Pultrusion molding involves pulling a substrate material through a bath containing the molding composition and then through suitable dies or fixtures which shape and cure the molding composition. Examples of molded articles produced by pultrusion, include, for example, plastic pipes, ladder rails, rods, and support beams such as I-beams. In preform molding, the molding composition is applied to a preformed substrate, usually fiberglass, having the shape of the desired article and cured in a heated matched die mold. Examples of molded articles produced by the preform process include, for example, automobile fenders and hoods, chairs and helmets. In wet mat molding, the molding composition is applied to a sheet of fiberglass mat, placed in a mold of a comparatively simpler geometry than a preform mold and cured. Examples of molded articles produced by wet mat processes include, for example, sheet stock for paneling, table tops and other articles of simple geometry.
Often, pigments are added to molding compositions in order to obtain a desired color in the final molded article. Two problems which are frequently encountered with pigmented molded articles are hazing, which is a lack of color depth, and mottling, which is non-uniform pigmentation.
Powdered polyethylene has been used as a shrink control additive in molding compositions in order to minimize hazing and mottling. However, polyethylene is not soluble in styrene, which is a common monomer used in molding compositions. This can lead to the filtering of polyethylene particles in the substrate materials, e.g., fiberglass, and an uneven distribution of the polyethylene throughout the mold. Moreover, polyethylene does not provide a high degree of shrinkage resistance in a molded part. As a result, molded articles with internally pigmented wet mix formulations which contain polyethylene often exhibit poor surface quality and warpage.
Poly(vinyl acetate) has been known to be an excellent shrink control additive for thermoset molding compositions, but generally has not been used as an additive in internally pigmented molding compositions because hazing and mottling can be severe. In addition, certain ethylene-vinyl acetate copolymers having a broad range of monomer content, e.g., from about 10 to 98 wt. % vinyl acetate, have also been proposed as additives in molding compositions. However, such ethylene-vinyl acetate copolymers have failed to provide a desirable balance of solubility in styrene, good pigmentation qualities, and good shrinkage resistance.
Accordingly, additive compositions comprising ethylene-vinyl acetate copolymers are desired which can provide good pigmentation qualities and shrink resistance as well as solubility in styrene. Improved molding compositions employing the ethylene-vinyl acetate copolymers as additives are also desired.